In the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) standard, a downlink transmission technology is based on Orthogonal Frequency Division Multiplexing (OFDM) and an uplink transmission technology is based on Single-Carrier Frequency Division Multiple Access (SC-FDMA).
The LTE system uses two types of frame structure, i.e., frame structure type 1 adopting Frequency-Division Duplex (FDD) and frame structure type 2 adopting Time Division Duplex (TDD). Frame structure type 2 includes seven kinds of frame structure configurations. The proportion of downlink sub-frames in each kind of frame structure configuration is fixed, ranging from 40% to 90%. As shown in FIG. 1, it can be clearly perceived from FIG. 1 that each radio frame consists of 10 radio sub-frames, and numbered sequentially from zero. Take configuration 0 for example:
Sub-frame 0 and sub-frame 5 are used for sending downlink data, i.e., the sub-frame 0 and sub-frame 5 are used by the Evolved NodeB (eNB) for sending information to the User Equipment (UE).
Sub-frames 2, 3 and 4 and sub-frames 7, 8 and 9 are used for sending uplink data, i.e., the sub-frames 2, 3, 4, 7, 8 and 9 are used by the UE for sending information to the eNB.
Sub-frame 1 and sub-frame 6 are known as special sub-frames, consisting of three special time slots. The three special time slots are respectively defined as Downlink Pilot Time Slot (DwPTS), Guard Period (GP) and Uplink Pilot Time Slot (UpPTS). The length of time of the DwPTS, GP, and UpPTS is variable. The specific value of the length of the time is configured by the system. The special sub-frames are used for sending the downlink data, and can be regarded as truncated downlink sub-frames.
The subsequent evolution of the LTE system is known as the “LTE-Advanced,” called LTE-A for short. An objective of the LTE-A is meeting system requirements of the International Mobile Telecommunications (IMT)-Advanced advanced by the ITU. A critical enhanced objective of the IMT-Advanced includes further enhanced data rate, interoperability/compatibility with other systems and worldwide roaming characteristics, etc. The objective of the data rate for downlink is 1 Gbps and the objective of the data rate for uplink is 500 Mbps.
Based on the above objectives, a concept of Carrier Aggregation (CA) is introduced into LTE version 10. The spectral efficiency of wireless resources is further improved by aggregating multiple continuous or discontinuous carriers into system bandwidth up to 100 Mhz and using the Multiple-Input Multiple-Output (MIMO) technology applied in the LTE-A uplink and downlink. The system of LTE version 10 is already able to meet the system requirements of the IMT-Advanced. However, in the actual network deployment and system operation, in most cases, competition of the spectrum and scattered available spectrum makes such a large-scale continuous spectrum aggregation unrealistic. In order to obtain the target peak rate of the system of LTE version 10, in the future, the system has to adopt the discontinuous spectrum allocation and bandwidth aggregation. While the discontinuous spectrum aggregation means that there are big differences between interferences of different frequency bands. Especially for the network deployment of the Time Division (TD)-LTE system, the interference between the uplink and downlink severely restricts performances of the TD-LTE system.
Based on the above analysis, in the future evolution of the TD-LTE system, an important issue to be considered in the evolution of the TD-LTE system is applying different frame structure configurations to different Component Carriers (CC)s. In the system of LTE Rel-10, when a UE is configured with multiple CCs, an eNB notifies the UE of the number of the Primary Component Carrier (PCC) and the number of aggregated Secondary Component Carriers (SCC)s through high-level signaling. In addition, when the multiple CCs configured for the UE are in different frequency bands, and the frame structure configuration of at least one CC is different from the frame structure of other CCs, how to design a timing relationship between a downlink data sub-frame Physical Downlink Shared Channel (PDSCH) and Uplink (UL) control information, and more specifically how to feed back the ACK/NACK becomes a key issue to be solved when the carrier aggregation between different bands adopts different frame structure configurations.